During the performance of a dynamic tracking or gated patient treatment in the field of radio therapy, a surrogate signal, such as a breathing signal, used to determine or correlate and extrapolate the target motion internal of the patient may change as the patient rests on the couch. A change may result from a repeated vital motion, such as breathing or heartbeat, but may also change in a more permanent manner, i.e. the change is not a repeated movement. This later change is primarily manifested as a shift called baseline shift. If a correlation model (or target-marker-model) is not altered or updated in order to adapt to this later change, the irradiation will likely miss the target. The change of a surrogate signal can take place while a patient is being treated.
In systems like Vero (of Brainlab) both X-ray imaging sources always have clear lines of sight on the target and thus, the correlation model can be easily updated using the 3D target coordinates obtained from stereoscopically taken X-ray images. The external markers can always (or most times) be detected. However, if a different system is used having a structure where one out of two X-ray imaging sources has no clear line of sight and is e.g. blocked, for example by a gantry, such as the ExacTrac (of Brainlab), the calculation of the correct 3D coordinates was impossible.